Abstract Body: Introduction
Prefrontal cortex (PFC) has long been thought to be involved in dual-tasking, and is also found to be associated with areas involved in motor control (supplementary motor area (SMA), premotor cortex). Functional near-infrared spectroscopy (fNIRS), a non-invasive neuroimaging technology, allows measurement of cortical activity during dual-tasking. This study aimed to compare cognitive-motor interference and brain activity between people with subcortical stroke and their healthy counterparts.

Methods
People with subcortical chronic stroke and their healthy counterparts (N=25/group; aged ≥ 50 years old, Montreal Cognitive Assessment score ≥ 22 points, Mini Balance Evaluation Systems Test score ≥16 points) were included. Each participant performed tasks in random order including: (1) single-task maintaining balance in standing position under vibration disturbance, (2) single-task serial-7-subtractions/clock test (while standing), and (3) dual-task maintaining balance in standing position in conjunction with a serial-7-subtractions/clock test. Participants were instructed to maintain balance as fast as they could safely while generating as many correct answers as possible during dual-task balance maintenance. Oxyhemoglobin (HbO) changes in bilateral PFC and SMA were measured with 46 channels of a continuous-wave system in fNIRS (ETG-4000, Hitachi). Path length of center of gravity was monitored by DynSTABLE system for postural sway. For both HbO and postural sway, dual-task cost was calculated as (dual-task performance-single-task performance) × 100%) / (single-task performance).

Results
People with subcortical stroke had greater dual-task cost for postural sway during dual-task serial-7-subtractions than their healthy counterparts (P<0.001). However, the dual-task cost for HbO changes was not significantly different between the two groups. There was moderate correlation of changes in oxyhemoglobin (HbO) in non-lesion hemisphere of PFC and postural sway during dual-task serial-7-subtractions among people with subcortical stroke (r = 0.538, P = 0.005).

Conclusions
PFC plays an important role in dual-task balance tests. People with subcortical stroke have similar brain activation compared to healthy older adults, but with worse dual-task performance. Dual-task serial-7-subtractions may serve as a better assessment to discriminate dual-task performance between people with subcortical stroke and their healthy counterparts compared to dual-task clock test.